Accusations that Syria used chemical weapons on rebel strongholds outside Damascus draw renewed attention to the banned nerve agent sarin – the weapon that medical personnel on the scene suspect was used and the agent US intelligence officials believe was used by government forces on at least two earlier occasions in the country’s civil war.

It was the earlier suspected uses of sarin in Syria that led President Obama to find in April that the regime of Bashar Assad had almost certainly crossed a “red line” in its 2-1/2-year battle with rebels seeking his ouster.

Perhaps the best-known recent use of sarin previously was in the 1995 Tokyo subway attack – known in Japan as the Subway Sarin Incident – in which members of a domestic cult-turned-terrorist group punctured bags of liquid sarin with sharpened umbrella tips in subway cars.

At least 13 people died in the attack and some 1,000 were injured.

But sarin’s legacy is about to get an update. With casualty reports suggesting the attacks near Damascus are the deadliest since Saddam Hussein gassed the Iraqi Kurdish village of Halabja in 1988 – sarin seems likely to be associated with Syria and Mr. Assad.

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What is sarin?

Syria's President Bashar al-Assad (R) speaks to a worker during his visit to the Umawyeen electricity station at Tishreen park on May Day in Damascus May 1, 2013, in this handout photograph distributed by Syria's national news agency SANA.

Sarin is a nerve agent first developed by German researchers in the late 1930s. Up to 500 times more toxic than cyanide, it is a colorless and odorless liquid that causes severe muscle spasms, vision loss, and asphyxia, and which can kill within a minute of contact in extreme cases.